Device for feeding reactor initiators

ABSTRACT

In a process for preparing polyethylene in tube reactors with or without autoclaves, where a free-radical initiator is introduced with or without cold ethylene into a flowing ethylene- and comonomer-containing medium, rotation is generated between two streams ( 61, 62 ) to be mixed at an angle ( 66 ) or by provision of a swirl element ( 20, 80 ) in the flow cross section ( 27, 28 ). In the region of a feed point ( 72, 81 ) for a free-radical initiator, there is provided a cross-sectional constriction ( 63, 67, 71 ) at which the free-radical initiator is introduced through an optimized off-center outlet opening ( 44 ) of an introduction finger ( 40 ) into the rotating flow ( 61, 62, 70 ).

The present invention relates to an apparatus for feeding initiator intoreactors, for instance feeding peroxide into high-pressure reactors forproducing LDPE.

Polyethylene (PE) is one of the most important plastics and has a highresistance to aqueous acids and alkalis. The plastic has good electricalproperties such as a low dielectric constant and a high specificresistance. Furthermore, this plastic combines good mechanicalproperties such as a high impact toughness with low densities, whichmakes it suitable for use in many technical fields. Thus, films andconsumer articles for domestic and industrial use are produced from PE;polyethylene is also employed for cable insulation and pipe sheathing.Low density polyethylene (LDPE) has a high transparency because of itslow crystalline content of only 50-70% compared to high densitypolyethylene (HDPE) in which the crystalline content is 70-90%, and thisfavors its use as a film material. A widely used method of producingpolyethylene films is calendering, by means of which polyethylene filmshaving thicknesses in the range from 0.05 to 1 mm can be produced. Incalendering, the thermoplastic is rolled out between many rolls betweenwhich the thermoplastic is molded to form an ever thinner film. Afterleaving the calender, the film is cooled on cooling rolls andsubsequently rolled up.

One process for preparing LDPE is the tube reactor process. At thebeginning of the polymerization, peroxide initiators are introduced inliquid form into the tube reactor. Compared to the amount of ethylene,the mass flow of the peroxide initiator is only small. A property of theinitiator used is that it quickly decomposes into free radicals underthe conditions prevailing in the tube reactor. To achieve a higheffectiveness of these initiators, for example peroxide, so as to ensurea high conversion, improved polymer properties and more stable reactoroperation, it is advantageous to mix the reactants with one another veryquickly.

EP 0 980 967 discloses a process for preparing ethylene homopolymers andcopolymers in a tube reactor at pressures above 1000 bar andtemperatures in the range from 120° C. to 350° C. by free-radicalpolymerization. Small amounts of free-radical initiators are firstlyintroduced into a flowing medium comprising ethylene, molar massregulators and optionally polyethylene, after which polymerizationoccurs. According to this process, the flowing medium is firstly dividedinto two volume elements flowing separately from one another and theseparately flowing volume elements are then set into relativecontrarotation by means of suitable flow-directing elements. Thecontrarotating, flowing volume elements are subsequently recombined toform a flowing medium and at the time of or shortly after thecombination of the contrarotating, flowing volume elements, thefree-radical initiator is introduced into the sheared boundary regionbetween the contrarotating flowing volume elements. EP 0 980 967 alsodiscloses an apparatus for carrying out this process. An improvement inmixing of the initiator metered in and, associated therewith, animprovement in the product quality was also able to be achieved byincreasing the flow velocity in the mixing zones.

The effectiveness of the free-radical initiator chosen depends on therapidity with which it can be mixed with the reaction medium initiallypresent in an individual case. For this purpose, injection fingers areused in industrial plants for the production of polyethylene.

EP 0 449 092 A1 describes the introduction of free-radical initiators,initiator mixtures or solutions of initiators in organic solvents viainjection fingers at a plurality of points along a reactor.

U.S. Pat. Nos. 4,135,044 and 4,175,169 describe how a comparativelysmall tube diameter in the initiation and reaction zones of ahigh-pressure reactor, relative to the enlarged tube diameter in thecooling zone, makes it possible to produce products having very goodoptical properties in high yields and at a relatively small pressuredrop over the length of the reactor.

Finally, U.S. Pat. No. 3,405,115 disclosed that uniform initiation ofthe polymerization reaction and optimum mixing of the reactioncomponents are of great importance for the quality of the polyethyleneobtained, for high reactor yields and for achieving uniform reactoroperation. According to this solution, initiators are mixed withsubstreams of cold ethylene in a special mixing chamber and only thenare introduced into the actual reactor. In the mixing chamber, the fluidin which the initiator does not decompose because of the lowtemperatures prevailing there is multiply diverted and passed throughchannels.

It is an object of the present invention to further optimize theintroduction of a free-radical initiator into a flowing medium so as togive as high a mixing speed as possible.

We have found that this object is achieved by a process for preparingpolyethylene in tube reactors and/or in combination with autoclaves, inwhich a free-radical initiator is introduced into a flowing ethylene-and possibly comonomer-containing medium and which comprises at leastthe following steps:

-   -   generation of rotation by mixing two streams to be mixed at an        angle or generation of rotation in the flowing medium by means        of swirl elements,    -   provision of a cross-sectional constriction with an inlet zone        upstream of the feed point for a free-radical initiator into a        reaction tube,    -   introduction of the free-radical initiator into the rotating        flow of the flowing medium and    -   provision of a downstream mixing zone and a cross-sectional        widening with an outlet.

The particular advantage of the method according to the presentinvention is that a more sparing introduction of free-radical initiatorcan be achieved by increasing the effectiveness of mixing. Thegeneration of rotation in the flowing medium increases the turbulencewhich results per se in an improvement in the effectiveness of mixing bymeans of transverse impulse exchange in the fluids to be mixed. Theprocess according to the present invention makes it possible to preparepolyethylenes which can be used to produce films having significantlyimproved optical properties, specifically in respect of transparency,because of lower proportions of high molecular weight material. Thesolution provided according to the present invention and the rapidmixing of the polyethylene-containing flowing medium with thefree-radical initiator enables significantly more stable reactoroperation at extraordinarily high maximum temperatures to be achievedwithout the final product tending to decompose. Furthermore, a fastertemperature rise in the reactor and a better low-temperature initiationbehavior of the polymerization when using initiators which decompose atlow temperature can be achieved. A further advantage of the processaccording to the present invention is the extremely short mixing-in timecompared to the half-life of the initiator.

In a further embodiment of the idea underlying the invention, the feedpoint for the free-radical initiator is located downstream of the pointat which rotation is generated in the flowing medium. This ensures thatthe free-radical initiator fed into the flowing medium at the feed pointalways enters a flowing medium which is already in a turbulent state, sothat the mixing time is reduced and the effectiveness of mixing issignificantly improved.

The geometry of the feed orifice of the element for feeding thefree-radical initiator into the rotating, flowing medium makes itpossible to influence the depth to which the free-radical initiator isinjected into the flowing medium. If the introduction orifice for thefree-radical initiator on the injection finger is made particularlysmall, a fine jet of the free-radical initiator can be injected verydeep, relative to the tube cross section, into the flowing medium.Depending on the flow velocity of the flowing medium, the injectiondepth of the free-radical initiator and thus the achievableeffectiveness of mixing can be positively influenced and matched bymeans of the geometry chosen for the feed orifice.

In one embodiment of the process according to the present invention, thefeed devices for substreams of the flowing medium are at an angle of 90°to one another. This enables a tangential flow component to be generatedin the resulting stream of the flowing medium and this flow componentgenerates circumferential rotation in the combined stream of the flowingmedium, which is desirable for achievement of turbulent flow. Before thesubstreams of the flowing medium are combined at an angle of 90° to oneanother, they can each pass through cross-sectional constrictions sothat the flow velocity can, depending on the ratio of the free toconstricted flow cross section, be doubled. If the substreams of therotating flowing medium are combined within the reaction tube, a furtherincrease in the turbulence of the combined flowing medium can beachieved by provision of a further cross-sectional constriction upstreamof the feed point for the free-radical initiator after passage throughan annular space.

The introduction of the free-radical initiator at the feed point ispreferably into a shear gap of the rotating flowing medium which rotatesin the circumferential direction in the flow cross section relative tothe position of the feed point for the free-radical initiator.

Another variant of the generation of a rotating flow comprises providingswirl elements in the free flow cross section over which the flowingmedium passes and by which the flowing medium is set into rotation inthe circumferential direction in the flow cross section, so that sheargaps arise.

Rotation in the flowing medium can be generated, on the one hand, insuch a way that a core stream is surrounded on its imaginary cylindricalouter surface, i.e. the shear surface, by an annular stream which hasbeen set into rotation relative to the core stream. The annular streamsurrounding the core stream can rotate either clockwise or anticlockwisearound the core stream. On the other hand, it is also possible to makethe core stream rotate and to generate rotation opposite to the rotationof the core stream in a stream surrounding the core stream.

The object of the present invention is also achieved by an apparatus forpreparing polyethylene in tube reactors, in which a free-radicalinitiator is fed into a flowing ethylene- and possiblycomonomer-containing medium and the flowing medium is conveyed through areaction tube having a changing flow cross section and a free-radicalinitiator is introduced in a mixing region of the reaction tube andeither substreams of the flowing medium impinge on one another at aparticular angle or swirl-generating elements are located in the flowcross section, with a feed element having an off-center inlet orificefor a free-radical initiator being located downstream of a constrictionin the rotating flow.

The apparatus according to the present invention for preparingpolyethylene is given tremendous mixing effectiveness by thefree-radical initiator being fed into shear gaps of a rotating flow,which have not only an axial flow component but also flow components inthe circumferential direction. Flow components in the circumferentialdirection effect impulse exchange transverse to the flow direction andthus provide the basis for effective mixing of a plurality of materials.

In a preferred embodiment of the apparatus of the present invention, theoutlet orifice at the tip of the feed element, which is configured as aflow-favorable injection finger is preferably inclined at 45° to theaxis of the finger. Depending on the cross-sectional diameter of theorifice, any angles in the range from 0° to 90° are possible. The swirlelements which are located in the free flow cross section in thereaction tube have, on their outer circumference, swirl blades whichextend over an annular space of the reaction tube by in each case about90° in the circumferential direction. In an alternative embodiment of aswirl element, the swirl blades are arranged on its outer circumferenceso that they extend over an annular space of the reaction tube by ineach case about 120° in the circumferential direction.

A further improvement in the mixing effectiveness can be achieved by theflow diameter in the region of the feed point for free-radical initiatorbeing reduced to about 70% of the free flow diameter. This enables theflow velocity to be increased by a factor of 2, which likewise makes agreat contribution to the effectiveness of mixing.

To avoid “deadwater” regions, the transition from the free flow crosssection upstream of the constriction to the latter forms a total angleof from 20° to 40°, so that an abrupt transition is avoided. The totalangle is particularly preferably 30°. To improve the mixing behavior,the diameter of the constriction downstream of the feed point for thefree-radical initiator is maintained over a mixing section length offrom about 10 to 20 tube diameters (D). After this mixing section offrom 10 to 20 tube diameters (D), the mixing section then widens at atotal angle of less than 20° back to the free flow cross section. Toprevent demixing phenomena in the transition from the narrower flowcross section to the wider flow cross section as a result of thedecrease in the velocity, the total angle is preferably less than 14°,so that a gradual transition from the mixing section cross section of0.7×D to D occurs.

The invention is described in more detail below with the aid of thedrawing.

In the drawing,

FIG. 1 shows an in-principle sketch of a mixing section with mixingregion and injection point for a free-radical initiator

FIG. 2 shows a swirl-generating component,

FIG. 3 shows a casing of the swirl element,

FIGS. 4 and 4.1 show an exterior swirl element,

FIGS. 5 and 5.1 show an interior swirl element,

FIGS. 6 and 6.1 show a flow-favorable injection finger,

FIG. 7 shows an injection point for a free-radical initiator locateddownstream of a swirl generator and upstream of a mixing section,

FIG. 8 shows a T-shaped connecting piece,

FIGS. 9, 9.1 and 9.2 show swirl-generating internals in flow crosssections with 90° and 120° blade configurations upstream of theinjection of a free-radical initiator.

FIG. 1 depicts an in-principle sketch of a mixing section with mixingregion and a feed point for a free-radical initiator.

The reaction tube 1 depicted in the schematic diagram of FIG. 1 can bepart of a tube reactor in which polyethylene LDPE is prepared by theprocess proposed according to the present invention. The reaction tube 1has an inlet cross section 2 and an outlet cross section 3. On the inletside, the reaction tube 1 is connected via a line system with a systemfor supplying reactants. Both a stream comprising fresh gas and theunreacted monomer recirculated via the high-pressure return circuit arefed into the mixing vessel 4 as fluctuation damper with buffer. Athrottle element 5 can be located upstream of the mixing vessel.Downstream of the mixing vessel 4, the reactant feed line is providedwith a compressor 6 by means of which the reactants, i.e. the flowingmedium going to the reaction tube 1, are compressed.

In the feed region 11, a free-radical initiator is fed via afree-radical initiator inlet line 7 into the interior of the reactiontube 1. For this purpose, a feed line system 7 via which a stock 8 of afree-radical initiator is supplied via a throttle element 9 and via acompressor 10 located downstream thereof to the feed point at which thefree-radical initiator, which initiates the polymerization reaction, isintroduced into the flowing medium in the reaction tube 1 is provided.The feed region 11 is followed in the flow direction 12 by a mixingregion 13 which preferably has a length of from 10× to 20× the diameter(D) of the reaction tube 1. The flowing fluid medium which is mixed inthe manner indicated below with the free-radical initiator introduced inthe feed region 11 passes through the mixing section 14.

The flow cross section of the reaction tube 1 is denoted by referencelabels 16 or D. The outlet end 3 of the reaction tube 1 is adjoined by apressure maintenance valve 15 by means of which the reaction mixtureobtained is depressurized. This results in phase separation.

In industrial plants for preparing LDPE, the pressure maintenance valve15 shown in the in-principle sketch of FIG. 1 serves as response valveand regulating valve. By means of this valve and a downstreamhigh-pressure separator 19.1, part of the flowing, ethylene-containingmedium is, on an industrial scale, returned after cooling to the plantvia a high-pressure circuit 19.3 and the LDPE obtained is passed to ahigh-pressure separator 19.1 from which the product 19.2 is subsequentlytaken off.

In industrial plants, the reaction tube 1 of a tube reactor is providedin the mixing region 13 and in the following mixing section 14 with wallcooling 18. The wall cooling 18 is usually configured as a coolingjacket which removes part of the heat of reaction evolved in thepolymerization reaction between the flowing medium and the free-radicalinitiator. The remainder of the heat of reaction remains in the flowingmedium. In addition, when the process of the present invention isemployed on an industrial scale, in which case a plurality of reactiontubes 1 each forming a reaction stage may be connected in series, themixing sections 14 can each be provided with cold gas inlet lines 17 a,17 b. Mixing-in a cold gas stream at the beginning of the mixingsections 14 allows a further part of the heat evolved in thepolymerization reaction to be compensated in the flowing mixture offlowing medium and free-radical initiators, which is relevant to theconversion. Furthermore, the free-radical initiator can be introducedinto the cold gas stream 17 b via the pump 10.

FIG. 2 shows a more detailed view of a swirl-generating component whichcan, for example, be installed in the reaction tube 1 shownschematically in FIG. 1.

The swirl element 20 depicted in FIG. 2 is accommodated in an outer tube22. The outer tube 22 in turn encloses an inner tube 23. On the outsideof the inner tube 23 there are located, as shown schematically in FIG.2, swirl-generating exterior blades 25 whose swirl blade area 36decreases in the direction of the outlet cross section 28 of the swirlelement 20. 2, 3, 4 or more exterior swirl blades 25 can be locatedopposite one another on the outer circumference of the inner tube 23.The interior of the inner tube 23 can, as shown in the embodiment inFIG. 2, be provided with an interior swirl blade 26. This gives the partof the stream passing through the interior cross section of the innertube 23 a rotational motion for generating turbulent flow, while thepart of the fluid medium passing through the annular space between innertube 23 and outer tube 22 is provided with a flow component in thecircumferential direction by means of the 2, 4 or more exterior blades25 located on the outer circumference of the inner tube 23. At theoutlet cross section 28 in the region of the points 34 of the swirlblades there is accordingly a rotating flow having a circumferentialcomponent relative to the center line 29.

FIG. 3 shows a casing of the swirl element depicted schematically inFIG. 2.

The casing of the swirl element 20 consists essentially of the outertube 22 which is located between two flanges 21. The inlet cross section27 is parallel to the outlet cross section 28 of the swirl element 20coaxial with the center line 29. The inner wall 30 of the outer tube 22represents the outer boundary of an annular gap which is formed betweenthe outer surface of the inner tube 23 and the outer tube 22 and throughwhich the exterior blades 25 which are fastened to the externalcircumference of the inner tube 23 pass in a screw-like fashion.

FIGS. 4 and 4.1 show an inner tube 23 provided with exterior bladeslocated opposite one another on the circumferential surface in greaterdetail.

The exterior blades 25 of which, in the embodiment depicted in FIG. 4,two are fixed opposite one another on the outer wall of the inner tube23 are attached to the inner tube 23 along a line of attachment 35. Theswirl blades 25 extend along the line of attachment 35 on the outersurface of the inner tube 23 in a screw-like fashion, with the screwline chosen here having a high pitch. It is also possible for more thanthe two exterior blades 25 shown in FIG. 4 to be provided on theexterior wall 33 of the inner tube, for example four or even six bladessymmetrically at 90° relative to the center line 29.

FIG. 4.1 shows a plan view of the rear part of the inner tube 23. InFIG. 4.1, the exterior blades 25 on the exterior wall 33 of the innertube are surrounded by the outer tube 22 of the swirl element 20. Inaddition, an interior swirl blade 26 which extends in a twisting fashionover a region of at least 90° along the inner wall of the inner tube 23is provided in the interior of the inner tube. This region can also beup to 180°. It is also possible for a plurality of flow channels to beformed.

FIGS. 5 and 5.1 show a side view of an interior swirl blade 26 and alsoa rear view thereof. Relative to its center line 29, the interior swirlblade 26 is provided with a twisted interior swirl blade surface 37which, as can be seen in FIG. 5.1, covers a 90° sector of the innersurface of the inner tube 23.

The screw-like pitches of the exterior blades 25 and the interior blades26 have the same sense; the exterior blades 25 and the interior blades26 can be fitted to a swirl element as shown in element 20 of FIG. 2with different pitches relative to one another. By means of thisconfiguration, the component of the flowing medium flowing through theinterior of the inner tube 23 can be given a counterclockwise rotationwhile the fluid component flowing between the exterior wall 33 of theinner tube and the inner surface 30 of the outer tube 22, i.e. in theannular space, has a clockwise rotation component imparted to it. It canbe seen from the details in FIG. 5 that all edges of the exterior andinterior swirl blades 25 and 26, respectively, which point in the flowdirection or in the direction opposite to the flow are streamlined toavoid eddy formation.

FIGS. 6 and 6.1 show a side view and plan view, respectively, of anintroduction element for the free-radical initiators, which ispreferably configured as a flow-favorable injection finger.

The introduction element is let into the wall of a reaction tube 1 andis provided with a cone tip 41. The introduction element 40 has a hole43 which, via a conical narrowing of the cross section, goes over into aconstricted hole which is adjoined by an outlet orifice 44 at an angle45. The angle of the outlet orifice 44 is, for example, 45° to the axisof symmetry of the feed element 40, with an angular range from 0 to 180°being possible, so that an oblique introduction of a free-radicalinitiator into a flowing medium can be achieved. The depth to which thefree-radical initiator penetrates into the rotating flowing medium canbe adjusted as a function of the angle and cross-sectional area of theoutlet orifice 44 and the flow of the cold gas stream 17, so that thedepth to which the free-radical initiator, e.g. peroxide, penetratesinto the flowing medium can be set independently of the degree ofturbulence generated. At the cone tip 41 of the finger-shaped feedelement 40, the outlet orifice 44 for the free-radical initiator ispositioned so that its circumference preferably enters a shear gap inthe rotating flowing medium. The parameters turbulence and injectiondepth of the free-radical initiator result in the high effectiveness ofmixing in the process proposed according to the present invention andthe apparatus for the preparation of polyethylene proposed according tothe present invention. The outlet orifice 44 on the cone tip 41 of thefeed element 40 is slightly offset from the center line of the feedelement 40. When injection is carried out without a cold gas stream 17,the angle is preferably from 0 to 15°. When a cold gas stream 17 isemployed, the angle is preferably 45° or can be selected within a rangefrom 30 to 60° to prevent the introduced stream from contacting thewall.

The flow-favorable injection finger 40 whose outlet orifice 44 points inthe flow direction of the flowing medium prevents the formation ofdeadwater regions downstream of it. This advantageously prevents regionsin which there are relatively high concentrations of the free-radicalinitiator forming as a result of eddies in the flow; such highconcentrations would otherwise lead to decomposition reactions whichhave a severe adverse effect on the product quality of the LDPE.

In place of the introduction of the free-radical initiator via theinjection finger 40, the initiator can also be introduced by means of acarrier medium. Thus, the free-radical initiator, e.g. peroxide, can beintroduced into the flowing medium in the cold gas inlet line 17 whichwould then have to be run, as shown in FIG. 1, into the injection region11 of the reaction tube. In place of cold gas as carrier medium for thefree-radical initiator, it is also possible to use cold ethylenebranched off immediately downstream of the compression stage 6 ascarrier gas for the free-radical initiator. If the free-radicalinitiator is introduced using cold gas as carrier gas, the cold gas andfree-radical initiator can be mixed in a mixing chamber and thispremixed stream can then be injected into the flowing medium at aconstriction, so that, when the introduction orifices and introductionangles are designed appropriately, a high impulse is achieved at thepoint of introduction.

FIG. 7 shows an injection point for a free-radical initiator, which islocated downstream of a swirl-generating element and upstream of amixing section.

A swirl element 20 with exterior swirl blades 25 is assigned to anorifice 51 which projects into a constricted flow cross section andthrough which a free-radical initiator is introduced into the flowingmedium. The exterior swirl blades 25 are located on the outer tube 22 ofthe swirl element 20 which has a length 87, preferably from about 1 to3×D. The swirl element 20 imparts a rotation to the flowing mediumwhich, after passing through a constricted cross section, enters theinjection region 11 for the free-radical initiator at an acceleratedvelocity.

In the embodiment shown in FIG. 7, the orifice 51 is at the end of atube 53 which is surrounded by a lens-shaped body 50 which isaccommodated between two sections of the reaction tube 1. Due to thepressure of the free-radical initiator, it is injected into the flowingmedium without contacting the inner wall 52 in the mixing region 11 ofthe reaction tube. After injection of the free-radical initiator intothe medium flowing in the flow direction 12, 24, the reacting mixtureenters a mixing section 14 which can be followed by a widening of theflow cross section not shown here.

In place of a feed point for pure free-radical initiator 72, 81, theinitiator can, in the embodiment shown in FIG. 7, also be introduced bymeans of a carrier medium, either cold gas 17 or an ethylene streambranched off upstream of the compression stage 6 (FIG. 1). Thefinger-shaped configuration of the introduction element 40 results inbeing formed no deadwater regions being formed downstream in the mixingregion 11, so that flow regions having a relatively high free-radicalinitiator concentration do not occur.

FIG. 8 shows a T-shaped connecting piece on a reaction tube in which tworeactant streams are mixed with one another.

On the reaction tube shown in FIG. 8, a first substream 61 and a secondsubstream 62 flow to an introduction point on the reaction tube at anangle 66. The first substream 61 of the reactant present as a flowingmedium passes through a first cross-sectional constriction 63 which isconfigured as a conical constriction 64 on the reaction tube. At anangle of 90° thereto, the second substream 62 of reactants flowsdownward in a vertical direction through a conical section 67 toward thereaction tube. Both substreams of the reactants present as flowing mediaexperience acceleration during passage through the respectivecross-sectional constrictions 63 and 67 before the second reactantstream experiences a deflection 66 of 90° and accordingly generates atangential flow 69. The tangential flow 69 occurs in the circumferentialdirection relative to the flow direction of the first substream 61,within an annular space 68 in the reaction tube 1. The substreams 61, 62of the reactant experience, due to the combination at an angle of 90°,mixing by introduction of a tangential flow component 69 into the fluidflowing along the reaction tube.

The fluid from the substream 62 in the annular space 68 in the reactiontube flows along the annular space 68 between the inner wall of thereaction tube and the outer wall of an insert element 65 and is combinedwith substream 61 at the end of the insert element 65. The combinedstream passes the feed point 72 for the free-radical initiator, e.g.peroxide, and a further cross-sectional constriction 71. Thecross-sectional constriction 71 is preferably configured so that thefree flow cross section at the feed point 72 for the free-radicalinitiator, for example peroxide, is preferably 0.7×D (free tubediameter). As a result, the rotating, accelerated and combined stream 70made up of the substreams 61 and 62 of the reactant is subjected tofurther acceleration. If the feed point 72 for the free-radicalinitiator on the tube wall is configured as a finger-shaped,flow-favorable injection element 40 as shown in FIGS. 6 and 6.1, afree-radical initiator is preferably introduced at shear gaps into therotating flow provided with a tangential flow component 69 so that rapidand effective mixing of the combined reactant stream is achieved. Thetotal angle at which the cross-sectional constriction 71 goes over fromthe original flow cross section D to 0.7×D is in the range from 20° to40°, particularly preferably a total angle of 30°.

The mixing section which follows the feed point 72 for the free-radicalinitiator preferably has a length of from 10×D to 20×D (D=tubediameter), but can also be 100×D, before there is, after the mixingsection, a transition to the original flow diameter D. The transitionfrom the mixing section diameter of 0.7×D to D preferably has, similarto a diffuser configuration, a total angle of from 10 to 20°,particularly preferably a total angle of less than 14°.

Another embodiment of the apparatus proposed according to the presentinvention for the preparation of polyethylene is shown in FIGS. 9.1 and9.2.

In these embodiments, the reactant stream 61 is conveyed as a singlestream to a cross-sectional constriction 41 [sic]. A division intosubstreams 61, 62 entering at inlet points at an angle to one another isnot provided for in this embodiment.

The constriction 71 goes over at a total angle of 30° into a narrowedflow cross section in a manner analogous to the embodiment depicted inFIG. 8. After passage through the constriction 71, the flow crosssection in the reaction tube is 0.7×D, which is maintained over themixing section which follows the feed point 81 for the free-radicalinitiator. The length of the mixing section is preferably from 10×D to20×D (D=original reaction tube diameter).

After the constriction 71, at which the flow velocity is increased by afactor of up to 2, swirl elements 80 are installed in the free flowcross section of the reaction tube. The swirl elements 80 are located,based on the flow direction 24, upstream of the feed point 81 for afree-radical initiator such as peroxide. In the embodiment depicted inFIG. 9.1, two swirl blades 82 are located on the outer circumference ofthe swirl elements 80. In this configuration, the swirl blades eachextend 90° around the external circumferential surface of the swirlelement 80 s, so that a rotation is imparted to the fluid stream whichenters at increased velocity. The ends of the swirl blades 82 fitted tothe outer surface of the swirl elements 80 touch the inside of thereaction tube 1 which encases the swirl elements 80. The edge 85 of theblades 82 on the outer surface 84 of the swirl elements 80 form a sealso that the fluid passing the swirl element 80 is forced through theannular space between outer surface 84 and inner wall of the reactiontube, thus ensuring generation of a flow component in thecircumferential direction during passage past the swirl element 80.

An alternative possible embodiment comprises, as shown schematically inFIG. 9.2, installing a swirl element 80 in the region of the reactiontube downstream of the constriction 71, with the swirl blades 82 fittedto the outer surface 84 of the swirl body 80 now extending 120° aroundthe circumferential surface 84 of the swirl element 80, as indicated byreference numeral 88. In this embodiment of the present invention, too,rotation is imparted to the reactant flow into which a free-radicalinitiator is to be introduced at the introduction point 81, as a resultof which the mixing conditions downstream of the introduction point 31for the free-radical initiator, e.g. peroxide, are significantlyimproved. The degree of turbulence can be influenced firstly by thepitch of the swirl blades 82 and by the length 87 of the swirl elements.Secondly, the achievable mixing effectiveness can be optimized by thedesign of the constriction 71 by acceleration of the reactant stream.

Significant parameters are, apart from the mixing parameters, the lengthof the mixing zone and the acceleration of the flowing medium.

An aspect common to the embodiments shown in FIG. 8 and FIGS. 9.1 and9.2 is that firstly the generation of rotation can be carried out onintroduction of the substreams 61 and 62 of the reactant, secondly arotating flow can be achieved by angled combination of the substreamsand thirdly rotation can be imparted to the fluid into which afree-radical initiator is to be introduced by means of the swirl element20, 80 located in the flow cross section. The introduction of thefree-radical initiator can be carried out either without or with coldethylene.

The internals employed according to the present invention for generatingrotation can also be retrofitted to existing plants after slightmodifications in order to increase their efficiency.

List of Reference Numerals

-   1 Reaction tube-   2 Inlet-   3 Outlet-   4 Mixing vessel-   5 Throttle element-   6 Compressor-   7 Inlet line for free-radical initiator-   8 Initiator reservoir-   9 Throttle element-   10 Compressor-   11 Injection region-   12 Flow direction-   13 Mixing region-   14 Mixing section-   15 Valve-   16 Flow cross section-   17 a Inlet line for cold gas-   17 b Inlet line for cold gas-   18 Wall cooling-   19 Fresh gas feed-   19.1 Separator-   19.2 Product-   19.3 High-pressure recirculation-   20 Swirl element-   21 Flange-   22 Outer tube-   23 Inner tube-   24 Flow direction-   25 Exterior swirl blade-   26 Interior swirl blade-   27 Inlet cross section-   28 Outlet cross section-   29 Center line-   30 Interior wall-   31 Exterior wall-   33 Exterior wall of inner tube-   34 Point of swirl blade-   35 Line of attachment-   37 Surface of interior swirl blade-   40 Injection finger-   41 Cone tip-   42 External screw thread-   43 Hole-   44 Outlet orifice-   45 Angle-   50 Injection lens-   51 Orifice-   52 Interior wall-   53 Tube-   60 T-piece-   61 First stream-   62 Second stream-   63 Cross-sectional constriction-   64 Conical section-   65 Insert-   66 90° dimension-   67 Conical section-   68 Annular space-   69 Tangential flow-   70 Constriction 61, 62-   71 Cross-sectional constriction for combined stream-   72 Injection of free-radical initiator-   73 Shear gap-   80 Swirl element-   81 Injection of free-radical initiator-   82 Swirl blade-   83 Extent of swirl blade 90°-   84 Outer surface of swirl element-   85 Edge of blade-   86 Annular space-   87 Length of swirl element-   88 Extent of swirl blade 120°

1. A process for preparing polyethylene in tube reactors and/or incombination with autoclaves, in which a free-radical initiator isintroduced into a flowing medium comprising ethylene and possiblycomonomers and which comprises at least the following steps: generationof rotation by mixing two streams to be mixed (61, 62) at an angle (66)or generation of rotation in a stream (61) by means of a swirl element(20), provision of a cross-sectional constriction (63, 67; 71) with aninlet zone upstream of the feed point (72, 81) for a free-radicalinitiator into a reaction tube (1), introduction of the free-radicalinitiator through an off-center outlet orifice (44) into the flowing,rotating medium (61, 62; 70) and provision in a downstream direction ofa mixing zone and a cross-sectional widening with an outlet.
 2. Aprocess as claimed in claim 1, wherein a plurality of reaction tubes (1)are connected in series and their mixing sections (14) are each assigneda main cold gas inlet line (17 a).
 3. A process as claimed in claim 1,wherein the heat of reaction is removed by means of wall cooling (18)and the introduction of cold gas (17).
 4. A process as claimed in claim1, wherein the free-radical initiator is fed to in injection region (11)by means of a carrier gas, cold gas main stream (17 a) or cold substreamof the flowing medium which has been branched off before compression. 5.A process as claimed in claim 1, wherein the feed point (72, 81) for thefree-radical initiator is located downstream of the point where rotationis imparted to the flowing medium (61, 62).
 6. A process as claimed inclaim 1, wherein the depth to which the free-radical initiator isinjected into the flowing medium (61, 62; 70) can be influenced by thegeometry of the outlet orifice (44) on the introduction finger (40). 7.A process as claimed in claim 1, wherein the feed facilities for theflowing medium (61, 62) are at an angle (66) to one another of from 45to 135°, but preferably 90°.
 8. A process as claimed in claim 1, whereinthe flowing media (61, 62) each pass through cross-sectionalconstrictions (63, 67) before they are combined.
 9. A process as claimedin claim 1, wherein the rotating, flowing medium (61, 62, 70) passesthrough a cross-sectional constriction (71) downstream of an annularspace (68) before reaching the feed point (72) for the free-radicalinitiator.
 10. A process as claimed in claim 1, wherein the free-radicalinitiator is fed into a shear gap (73) of the rotating flowing medium(70) at the feed point (72).
 11. A process as claimed in claim 1,wherein the rotation in the flowing medium (61, 62, 70) is generated bymeans of swirl elements (20, 80) located in the flow cross section (27,28) up-stream of the feed point (72, 80).
 12. An apparatus for preparingpolyethylene in tube reactors, in which a free-radical initiator is fedinto a flowing ethylene- and possibly comonomer-containing medium (61,62) and the flowing medium (61, 62) passes through a reaction tube (1)having a changing flow cross section (27) and a free-radical initiatoris introduced in a mixing region (13), wherein substreams (61, 62) ofthe flowing medium impinge on one another at an angle (66) or swirlelements (20, 80) are located in the flow cross section (27, 28) and afeed finger (40 having an off-center outlet orifice (44) for a freeradical initiator is located downstream of a constriction (71) in therotating flow (70).
 13. An apparatus as claimed in claim 12, wherein theoutlet orifice (44) at the tip (41) of the feed finger (40) is inclinedto the axis of the latter at an angle of from 5 to 80°.
 14. An apparatusas claimed in claim 12, wherein the swirl elements (20, 80) are providedon their outer circumference with swirl blades (25, 82) which eachextend over from 45 to 360°, in the circumferential direction in anannular space (68) of the reaction tube (1).
 15. An apparatus as claimedin claim 12, wherein the swirl elements (20, 80) are provided on theirouter circumference with swirl blades (25, 82) which each extend overfrom 45 to 360°, in the circumferential direction in an annular space(68) of the reaction tube (1).
 16. An apparatus as claimed in claim 12,wherein the diameter of the constriction (71) is from about 0.2 to 0.95times, the diameter D of the free flow cross section (27, 28).
 17. Anapparatus as claimed in claim 12, wherein the free flow cross section(27) upstream of the constriction (71) goes over at a total angle offrom 10° to 70° into the constriction (71).
 18. An apparatus as claimedin claim 17, wherein the total angle is 30°.
 19. An apparatus as claimedin claim 12, wherein the diameter 0.7×D of the constriction (71)downstream of the feed point (71, 82) for the free-radical initiatorremains unchanged over a mixing section length (13) of from 10×D to100×D.
 20. An apparatus as claimed in claim 12, wherein the mixingsection (13) goes over after from 10×D to 100×D into the free flow crosssection diameter D at a total angle of less than 20°.